This invention concerns a zero-crossing point detection circuit.
Zero-crossing point detection circuits are well known for detecting the instances at which a signal voltage changes from a negative level to a positive level and from the positive level to the negative level, such as in FM demodulators, phase synchronization circuits, bit timing information extraction circuits in data transmission, or the like.
Heretofore, such zero-crossing point detection circuits have included a level comparator consisting of a differential operational amplifier of high gain, a differentiation circuit and a full-wave rectification circuit. Further, with the development of digital circuits in recent years, a combination of a delay circuit and an EXCLUSIVE OR circuit has been used in some circuits in place of the differentiation circuit and the full-wave rectification circuit.
FIG. 1 shows one example of the former and FIG. 2 shows one example of the latter. The zero-crossing point detection circuit shown in FIG. 1 comprises a level comparator 1, a differentiation circuit consisting of a capacitor C1 and a resistor R1, and a full-wave rectification circuit consisting of a transformer T1 and diodes X1 and X2. An input signal shown in FIG. 3(a) is inputted into the level comparator 1 to be processed as shown in FIG. 3(b), then differentiated in the differentiation circuit into the form shown in FIG. 3(c) and processed in the full-wave rectification circuit into the waveform as shown in FIG. 3(d), which waveform then appears across an output resistor R2.
On the other hand, the zero-crossing point detection circuit exemplified in FIG. 2 comprises a level comparator 1, a pulse delay circuit 2 connected to the output of the comparator, and an EXCLUSIVE OR circuit 3 for receiving the output from the level comparator 1 and the output from the pulse delay circuit 2. The input signal is processed in the level comparator 1 into the waveform as shown in FIG. 3(b), delayed in a delay circuit 2 as shown in FIG. 3(e) and, as a result, outputted from the EXCLUSIVE OR circuit as pulses shown in FIG. 3(f).
While these conventional zero-crossing point detection circuits are advantageous in that they can operate even when the level for the input signals is low, this advantage is accompanied by the disadvantage that they operate in response to unnecessary small signals such as noise which may occur in the absence of input signals. In order to overcome the foregoing disadvantage, the use of a Schmitt circuit has been considered, but in such a case, the output from the circuit will be displaced from the exact time points of the zero-crossing, and AM components in the input signals appear as phase jitter in the output.